Fixed crash where collection children are ID blocks instead of GR blocks

Not sure when/how that happens, but it happened on a lighting file of the
Spring project, when linking in a nested collection of which parts where
also linked in from other blend files.

blender_asset_tracer/blendfile/__init__.py

@@ -498,9 +498,13 @@ class BlendFileBlock:
     def dna_type(self) -> dna.Struct:
         return self.bfile.structs[self.sdna_index]
 
+    @property
+    def dna_type_id(self) -> bytes:
+        return self.dna_type.dna_type_id
+
     @property
     def dna_type_name(self) -> str:
-        return self.dna_type.dna_type_id.decode('ascii')
+        return self.dna_type_id.decode('ascii')
 
     @property
     def id_name(self) -> typing.Optional[bytes]: